Differential interferometer



May 1954 D. R. BUCHELE ET AL DIFFERENTIAL. INTERFEROMETER 3 Sheets-$heet 1 Filed Oct. 10, 1950 DONALD R. BUGHELE P/ERUE B. DAY

M y 2 1954 D. R. BUCHELE ETAL 2,679,183

DIFFERENTIAL INTERFEIROMETER Filed Oct. 10, 1950 3 Sheets-Sheet 2 DONALD R. BUDHELE PIERCE D. DAY

y 1954 D. R. BUCHELE ET AL 2,679,183

' 4 DIFFERENTIAL INTERFEROMETER Filed Oct. 10, 1950 3 Sheets-Sheet 3 72 2/ FIG. 7 f u 7 7 w DONALD E. BUOHELE PIERCE 8. DAY

Patented May 25, 1954 attain DIFFERENTIAL INTERFEROMETER Donald R. Buchele, Cleveland, Ohio, and Pierce B. Day, Rochester, N. 2.

Application October 10, 1950, Serial No. 189,464

8 Claims. (Cl. 8814) (Granted under Title 35, U. S. Code (1952),

sec. 266) The present invention relates to a differential interferometer and more particularly to a differential interferometer for measuring optical path differences by optical interference. Although not limited thereto, the interferometer of the present invention finds its chief application in the measurement of air density in wind tunnels, and the following description will relate principally to that application.

The application of the Mach-Zehnder interferometer to air density measurements has demonstrated the essential validity of the interference method in aerodynamics, and interferometers of this type have become basic working tools for aerodynamic research. There are, however,

practical limits to the size of the Mach-Zehnder interferometer and the field which it will cover. In particular, the glass splitter plate used in this interferometer must be the same size as the field of view, and must be thicker than the tunnel windows.

With the advent of larger wind tunnels, it has become increasingly diflicult to obtain sufficiently large splitter plates having the optical qualities essential to interferometer use. In addition, the large-size optical components required by this interferometer for present day use entail a massive unwieldy supporting structure. Finally, these components and supporting structure render the interferometer highly sensitive to vibra tion, which is ever present in wind tunnel operations.

These basic limitations to large-size interfere meters have been recognized by the prior art,

and attempts have been made to eliminate them.

One interferometer has been developed which offers a large working field of parallel light and employs small splitter plates. However, this interferometer, in its simplest form, contains eight mirrors having an aperture equal to the working field, and is less compact than the Mach- Zehnder interferometer it is designed to replace.

The present invention proposes an interferometer of the differential type which eliminates the former limits of practical size of field, and makes interferometer installations in large wind tunnels both practicable and within reasonable economical consideration. The interferometer of the present invention, in one embodiment, is applied to the two-mirror schlieren system which is unquestionably the best for large-size installations, in both optical performance and structural simplicity.

In particular, the present invention proposes an interferometer in which the light passing through the wind tunnel is separated into two parallel beams, a test beam in which the model under test is placed, and a reference beam which can be made to interfere with the test beam. Interference is caused by superposing the two half-circles of the beams, the half-circles being moved so that coherent rays coincide. The maximum size of the working field is therefore somewhat less than one-half the window aperture and the mirror aperture.

Accordingly, an object of the present invention is the provision of a differential interferometer for measuring optical path differences.

A further object is to provide a differential interferometer which is simple and small but possesses a large field of view.

Another object is the provision of a differential interferometer in which the splitter plates may be a small fraction of the size of the field of view.

A'still further object of the invention is to provide a differential interferometer which is less sensitive to vibration than conventional interferometers.

Still another object is to provide a differential interferometer which may utilize existing components of standard schlieren apparatus.

Other objects and advantages of the invention will hereinafter become more fully apparent from the following description of the annexed drawings wherein:

Fig. l is a schematic diagram, partly in section of one form of the present invention;

Figs. 2a to 20 are cross-sections of the light beams of the present invention taken along the line 2-2 of Fig. 1 at various times during the operation of the same;

Figs. 3a to 30 are views similar to Fig. 2 taken along the line 3-3 of Fig. 1;

Fig. 4 is a schematic diagram of the receiver unit of Fig. 1, illustrating the mode of adjustment of this unit for fine fringes;

Fig. 5 is a schematic diagram, partly in section, of another form of the present invention for measuring absolute densities;

Fig. 6 is an enlarged sectional view taken along the line 6-6 of Fig. 5; and

Fig. 7 is a schematic diagram, partly in section, of still another form of the present invention employing a single spherical mirror and a single conversion unit.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in Fig. l, which illustrates a preferred embodiment of the present invention as applied to a typical two-mirror schlieren system, a portion l l of a conventional wind tunnel having a pair of aligned viewing windows l2 at an intermediate position therein. Positioned on opposite sides, respectively, of portion i! and in alignment with windows 52 are a pair of parabolic reflectors or mirrors :3, whose focal points are represented at 14.

The other portions of the interferometer according to the invention may be considered as comp ising a source conversion unit, generally designated 65, positioned exteriorly and on one side or portion ii, and a receiver conversion unit, generally designated it, positioned exteriorly and on the other side of portion H. Source conversion unit to comprises a source or" light il, a splitter plate it, a pair of front-surface mirrors 5!; and 2!, and one semi-disc frontsurface mirror 22. Receiver unit it comprises a light sensitive device 23, such as a camera, a pair of front-surface mirrors 2:? and 25, a semidisc front-surface mirror 26, and a splitter plate 2?. It is thus seen that source conversion unit and receiver conversion unit it are essentially identical, except that mirror is provided with rotational and translational adjustability, splitter plates is and El also being rotatable, for the purpose described below.

Conversion unit it is arranged so that all four elements 28, iii, '2 i, and 22 are mutually parallel and at 45 to the center line 223 joining the focal point it and the center of one parabolic mirror 13. Conversion unit it is arranged similarly with respect to center line 2t between the other parabolic mirror is and its focal point It, with the reflecting surfaces of splitter plates i3 and 21 being located at the focal distances from their respective parabolic mirrors l3. Now, if the light source ii is imaged at the surface of plate 28 by means of any suitable lens 3!, half the surface of mirror i3 is illuminated by light reflected at plate it and then reflected at mirrors [9 and 22, this beam of light being designated 32. The other half of the surface of mirror 53 is illuminated by light transmitted through plate 58 and reflected at mirror 2 l this beam being designated 3S.

Beams 32 and 33 are then reflected by mirror it in parallel through one window It, portion H. and the other window E2 to the other parabolic mirror 13. Beam 32 is then reflected by mirror [3 to mirror 25, from which it is reflected through splitter plate 2'? to camera 23. Beam 33 is reflected from mirror It, and from mirror" 25 and 24, to plate 2'5, from which it is reflected to camera 23. The cross-section of beams 32 and as viewed along lines 2 and 33 of Fig. l, are shown in Figs. 2a and 3a. It is thus seen that with this initial setup there is no overlapping of beams 32 and 33, and there are no coherent rays and no interference.

To obtain interference between beams 32 and 33, splitter plate i3 is rotated through an angle :c/2, where a: is one half the aperture ratio of the parabola of parabolic mirror l3. After rotation of plate It through this angle, beams 32 and 33 are derived from the same part of the one beam from source ll incident upon plate it. Any ray in this beam incident upon plat [8 is, split into two coherent rays of lesser intensity, one such ray 34 entering beam 32 and the other such ray 35 entering beam Beams 32 and 33 are now coherent and can be made to interfere, the cross sections of the beam being indicated in Figs. 2b and 312. By rotating splitter plate 21 through angle :c/Z, the two coherent beams are made to overlap in the image of camera 23, the optical path dicerence between the beams being made zero by translation of mirror 25. The cross-sections of beams 32 and and the location of coherent rays 3 and and a second pair of coherent rays 38 and 37, are indicated in Fi s. 2c and 3c.

The adjustments of plates is and 2'1 to obtain the beam arrangements shown in Figs. 2c and ma be termed the infinite fringe adjustment (a fringe of infinite width), and it is then necessary to adjust unit it further in order to obtain many fringes which are in focus at th virtual center of the plane of portion N. This further adjustment of unit it is made by rotating mirror 25 and plate 2? in the manner set forth below.

Referring now to Fig. 4:, the various elements oi unit it are shown in their positions for infinite fringe adjustments, coherent rays 34 and being shown in this condition as dotted lines.

mirror 25 is rotated from its dotted line position through an angle 11/2 to th solid line position, designated ray 34 will, now be reflected by mirror 25 to a position designated 3t. Rays and Si:- will now intersect at mirror '55, a distance a from plate 2?. To move this point of intersection to the plane 38 of tunnel H, plate 21 is rotated from its dotted line position through angle 2/). to its solid line position designated 2?. Thus, ray will now be reflected by plate 2? to a position and rays 35 and 35 will intersect at an angle 20, the point of intersection being a distance d from plate ill.

From the geometry of Fig. 4, and under the assumption that angles 10, y and z are very small, it is readily apparent that and Therefore, substituting f0 20 in the first equation,

and

Since the distance d is usually much greater than a, approximate equal rotation of mirror 25 and plate thermore, in most large twocnirror schlieren systems, tunnel H is at the focal distance from mirror is, optical distance d is infinite and angles .2 and y are equal. The angle 20 is the measure of the spacing of the virtual fringes in tunnel I 4.

With the light path shown in Fig. 4, it is seen that the direction of rays 35 and 3E emerging from plate 2! has changed with the adjustment for fine fringes. This change will cause the image to shift slightly across camera 23. Actual tests have shown that the shifting of the image is not sumcient to be objectionable, and does not in any way detract from the value of the attained results. It is also possible to eliminate the shift ing by using the alternate beam emergin from plate 2?.

Referring now to Fig. 5, there is shown a modification of the system of Fig. l which would permit the measurement of absolute densities in medium-size tunnels. There is shown in Fig. 5 a portest portion H of the tunnel.

tion 5! of a wind tunnel having windows 52, a source conversion unit 55, a receiver conversion unit 56, and a pair of parabolic mirrors 53. In this arrangement, the reference beam 63, which is the lower beam, passes around tunnel 5!, tunnel 5i being constructed with one relatively thin wall for a minimum loss of field, as shown in Fig.

6, test beam at being indicated therein.

a As shown in Fig. 6, mirror 53 has a diameter of twice the value as that of windows 52. Since actual laboratory experience indicates such arrangements are feasible, it is apparent that this requirement of the present invention is justiflable from a practical standpoint.

In Fig. '7 there is shown another embodiment 'of the present invention which employs a single conversion unit '75 and a single spherical mirror 13 positioned in alignment with windows '12 in In this embodiment, mirror 13 and unit I5 are positioned on opposite sides, respectively, of portion I I the center of curvature of mirror 13 being indicated at '14,

As shown in Fig. '7, conversion unit 15 comprises a splitter plate it, a pair of front-surface mirrors Ti, 73, and a semi-disc front-surface mirror 2'9, each of these elements of unit 15 being mutually parallel and. at to the center line joining the focal point M and the center of the spherical mirror 13. A light source 8! is imaged at the center of plate 16 by means of any suitable lens 82, source 8! being positioned at an angle x/2 to the right of the center of plate it, where x is one-half the ratio of diameter to radius of curvature of the spherical mirror 13. It is seen that a portion of the light entering plate 16 is transmitted therethrough, reflected by mirror ll and reflected by mirror 19 to form beam 83. If plate '16 is now rotated through angle 02/2, a portion of the light enterin plate 16 is reflected thereby and by mirror 78 to form beam 84. Furthermore, since the common center of beams 83 and 34 is positioned at the center of curvature 14 of mirror F3, beams 83 and 84 will pass through portion H to mirror l3 and be reflected back on themselves by mirror is to plate l6.

A camera 85 is positioned adjacent to plate 16 and is focused so that 84 enters therein by transmission through plate '56 and beam 83 enters therein by reflection from plate it. It is apparent that this embodiment of the present, invention may also be used for absolute path lengths and density measurements by passing reference beam 8% around the test portion H, and by positioning mirror 13 so that portion H covers onehalf the surface thereof.

It is thus seen that the present invention provides an interferometer of the differential type in which the optical splitter plates used may be a small fraction of the size of the field of view. As a result, the interferometer is relatively simple and inexpensive and less unwieldy than those of the prior art. Furthermore, th interferometer may use existing schlieren mirrors and tunnel windows as components'thereof when operated as atwo mirror system. Finally the conversion units are intend-ed to be assembled as a unit along with the source of light, or the camera, or both,

as the case may be, thus requiring a single supporting structure, and making the interferometer less sensitive to vibration.

It should be understood of course, that the foregoing disclosure relates to only preferred embodiments of the present invention and that numerous modifications or alterations may be made ill) 6. therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In combination with a wind tunnel for measuring the air density about a model therein, said tunnel having a test portion including a pair of aligned windows at opposite sides thereof, a differential interferometer comprising a source of light, light-sensitive means, splitter plate means, first opaque mirror means, second opaque mirror means, third opaque mirror means, and a reflector, said first, second and third mirror means being mutually parallel, said splitter plate means dividing the light from said source into a pair of coherent beams, one of said beams being formed by reflection of the light by said splitter means, the other of said beams being formed by transmission of the light by said splitter means, said one beam being reflected from the splitter means to the reflector by said first and third mirror means, said other beam being reflected to said reflector from said splitter means by said second mirror means, said coherent beams diverging from their respective mirrors to said reflector in side by side relationship, means including said reflector for directing one of said beams through the test portion across the model and thereafter directing the beam to said light-sensitive means, and means including said reflector for directin the other of said beams to said light-sensitive means in overlapping relationship with the first directed beam.

2. For use with a wind tunnel having a test portion, a differential interferometer comprising a source of light, light-sensitive means, splitter plate means, first opaque mirror means, second opaque mirror means, third opaque mirror means, and a reflector, said first, second and third mirror means being mutually parallel, said splitter plate means positioned in the path of the light from said source for dividing the light into a pair of coherent beams, one of said beams being formed by reflection of the light by said splitter means, the other of said beams being formed by transmission of the light by said splitter means, said one beam being reflected from the splitter means to the reflector by said first and third mirror means, said other beam being reflected to said reflector from said splitter means by said second mirror means, said coherent beams diverging from their respective mirrors to said reflector in side by side relationship, means including said reflector for directing one of said beams through the test portion and thereafter directing the beam to said light-sensitive means, and means including said reflector for directing the other of said beams to said lightsensitive means in overlapping relationship with the first directed beam.

3. In combination with a wind tunnel for measuring the air density about a model therein, said tunnel having a test portion including a pair of aligned Windows at opposite sides thereof, a differential interferometer comprising a source of light, light-sensitive means, splitter plate means, first opaque mirror means, second opaque mirror means, third opaque mirror means, and a reflector, said first, second and third mirror means being mutually parallel, said splitter plate means dividing the light from said source into a pair of coherent beams, one of said beams being formed by reflection of the light by said splitter means, the other of said beams being formed by transmission of the light by said splitter means, said one beam being reflected from the splitter means to the reflector by said firsu and third mirror means, said other beam being reflected to said reflector from said splitter means by said second mirror means, said coherent beams diverging from their respective mirrors to said reflector in side by side relationship, and means including said reflector for directing said beams in parallel relationship through said Windows and thereafter directing said beams to said light-sensitive means in overlapping relationship.

4. In combination with a wind tunnel for measuring the air density about a model therein, said tunnel having a test portion adjacent up stream reference portion including a pair of aligned windows at opposite sides thereof, a differential interferometer comprising a source of light, light-sensitive means, splitter plate means, first opaque mirror means, second opaque mirror means, third opaque riairror means, and a reflector, said first, second third mirror means being mutually parallel, said sp 'tter plate means dividing the light from said source into a pair of coherent beams, one of said beams being formed by reflection of the light by said splitter means, the other of said beams being formed by transmission of the light by said splitter means, said one beam being reflected from the splitter means to the reflector by said first and third mirror means, said other beam being reflected to said reflector from said splitter means by said second mirror means, said coherent beams diverging from their respective mirrors to said reflector in side by side relationship, means including said reflector for directing one of said beams through the test portion across the model and thereafter directing the beam to said light'sensitive means, and means including said reflector for directing the other of said beams parallel to said first beam and across the reference portion thereafter to said light sensitive means in overlapping relationship with the first directed beam.

5. For use with a wind tunnel having a test portion, a differential interferometer comprising a source of light, light-sensitive means, splitter plate means positioned in the path of the light from said source for dividing the light into a pair of coherent beams, one of said beams being .tormed by reflection of the light by said splitter means, the other of said beams being formed by transmission of the light by said splitter means, said one beam being reflected from the splitter means to a first reflector by first mirror means, said other beam being reflected to said first refiector from said splitter means by second mirror means, said coherent beams diverging from their respective mirrors to said reflector in side by side relationship and being reflected parallel to each other to a second reflector, and means including the second reflector for directing the beams in overlapping relationship to said light-sensitive means.

6. The combination of claim 5, wherein said last mentioned means includes a splitter plate positioned between said second reflector and light-sensitive means.

7. In combination with a Wind tunnel for measuring the air density about a model therein, said tunnel having a test portion including a pair of aligned windows at opposite sides thereof, a differential interferometer comprising a source of light, light-sensitive means, splitter plate means, first opaque mirror means, second opaque mirror means, third opaque mirror means, and a reflector, said first, second and third mirror means being mutually parallel, said splitter plate means dividing the light from said source into a pair of coherent beams, one of said beams being formed by reflection of the light by said splitter means, the other of id beams being formed by transmission of the light by said splitter means, said one beam being reflected from the splitter means to the reflector by said first and third mirror means, said other beam being reflected to said reflector from said splitter means by said second mirror means, said coherent beams diverging from their respective mirrors to said reflector in side by side relationship, means including said reflector for directing one of said through the test portion across the model and thereafte directing the beam to said light-sensitive means, and means including said reflector for directing the other of said beams parallel to said first beam and exteriorly of said tunnel and thereafter to said light-sensitive means in overlapping relationship with the first directed beam.

8. In combination with a Wind tunnel for measuring the air density about a model therein, said tunnel having a test portion and adjacent upstream reference portion including pair of aligned windows at opposite sides thereof, a differential interferometer comprising a source of light sensitive means, splitter plate means for dividing the light from said source into a pair of coherent beams, one of said beams being formed by reflection of the light by said splitter means, the other of said beams being formed by transmission of the light by said splitter means, said one beam being reflected from the splitter means to a reflector by first mirror means, said other beam being reflected to said reflector from said splitter means by second mirror means, said coherent diverging from their respective mirrors to said reflector in side by side relationship, means including said reflector and a second reflector aligned therewith for directing one of said beams through the test portion across the model and the other of said beams parallel to the one beam and across the reference portion and thereafter directing said beams to the light-sensitive means in overlapping relationship.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,256,804 Hurley Sept. 23, 1941 2,256,855 Zobel Sept. 23, 1941 2,434,929 Williams Jan. 5, 1948 2, 33,244 Stanun Sept. 27, 1949 FOREIGN PATENTS Number Country Date $9,545 Germany June 10, 1930 509,316 Germany Oct. 8, 1930 386,315 Great Britain Jan. 12, 193 2 577,377 Germany July 12, 1933 383,076 France is Mar. 15, 1943 

